Image recording systems write digital data onto photosensitive media by applying light exposure energy. Such energy may originate from a number of different sources and may be modulated in a number of different ways. Image recording systems can be used for digital printing, whereby digital image data is used to print an image onto photographic paper or film. This invention specifically relates to the high speed (multiple frames per second) writing of digital image data onto 35 mm color movie film.
One of the early methods used for digital printing onto movie film was cathode ray tube (CRT) based systems. In a CRT-based printer, the digital data is used to modulate the CRT, which provides exposure energy by scanning an electron beam of variable intensity along a phosphorescent screen. This technology has several limitations related to the phosphor and the electron beam. The resolution of this technology is limited to approximately 1000 pixels across the film, perforation to perforation, which roughly corresponds to 1000 DPI (dots per inch). CRT printers also tend to be expensive, which is a severe shortcoming in cost sensitive markets such as photoprocessing and film recording. An additional limitation is that CRT printers can only operate at rates of about one minute per frame. Although this may be acceptable for small segments of special effects, it is far too slow for digital editing and enhancement of entire feature length films.
Another commonly used approach to digital printing is the laser-based engine shown in U.S. Pat. No. 4,728,965. Digital data is used to modulate the duration of laser on-time or intensity as the beam is scanned by a rotating polygon onto the imaging plane. Such raster scan systems use red, green, and blue lasers. Unfortunately, as with CRT printers, the laser based systems tend to be expensive, since the cost of blue and green lasers remains quite high. Additionally, compact lasers with sufficiently low noise levels and stable output so as to allow for accurate reproduction of an image without introducing unwanted artifacts are not widely available. Commercially available laser scanner systems tend to write images onto movie film in the range of 3 to 10 seconds per frame and have been used primarily for special effects lasting only tens of seconds. For digital mastering of full length feature films, a throughput of about 2 frames per second is needed.
Another problem with laser based printing system is that both photographic paper and film are often not suitable for a color laser printer due to reciprocity failure. High intensity reciprocity failure is a phenomenon by which both photographic paper and film are less sensitive and have reduced contrast when exposed to high light intensity for a short period. For example, raster scan laser printers expose each of the pixels for tens of nanoseconds, whereas optical printing systems expose the paper for the duration of the whole frame time, which can be on the order of a second. Thus, special paper and film are required for laser printers.
In an effort to reduce cost and complexity of printing systems, avoid reciprocity failure, and increase the throughput of the writer, alternative technologies have been considered for use in digital printing. Among suitable candidate technologies under development are two-dimensional spatial light modulators. Two-dimensional spatial light modulators, such as the digital micromirror device (DMD) from Texas Instruments, Dallas, Tex., or liquid crystal devices (LCD) can be used to modulate an incoming optical beam for imaging. A spatial light modulator is essentially as a two-dimensional array of light-valve elements, each element corresponding to an image pixel. Each array element is separately addressable and digitally controlled to modulate light by transmitting or by blocking transmission of incident light from a light source. A liquid crystal spatial light modulator does this by changing the polarization state of light. Polarization considerations are, therefore, important in the overall design of support optics for a spatial light modulator.
There are two basic types of LCD spatial light modulators in current use. The first type developed was the transmission spatial light modulator, which, as its name implies, operates by selective transmission of an optical beam through individual array elements. The second type, a later development, is a reflective spatial light modulator. The reflective spatial light modulator, operates by selective reflection of an optical beam through individual array elements. A suitable example of an LCD reflective spatial light modulator relevant to this application utilizes an integrated complementary metal oxide semiconductor (CMOS) backplane, allowing a small footprint and improved uniformity characteristics.
Spatial light modulators provide significant advantages in cost, as well as avoiding reciprocity failure. Spatial light modulators have been proposed for a variety of different printing systems, from line printing systems such as the printer depicted in U.S. Pat. No. 5,521,748, to area printing systems such as the system described in U.S. Pat. No. 5,652,661.
A single spatial light modulator such as a Texas Instruments digital micromirror device (DMD) as shown in U.S. Pat. No. 5,061,049 can be used for digital printing applications. One approach to printing using the Texas Instruments DMD, shown in U.S. Pat. No. 5,461,411, offers advantages such as longer exposure times compared to laser/polygon writers. Thus, the reciprocity problems associated with photosensitive media during short periods of light exposure are eliminated. However, DMD technology is both expensive and not widely available. Furthermore, DMDs are not easily scaleable to higher resolutions, and the currently available resolution is not sufficient for all digital printing needs.
Several photographic printers using commonly available LCD technology are described in U.S. Pat. Nos. 5,652,661; 5,701,185; and 5,745,156. Most of these designs involve the use of a transmissive LCD modulator such as those depicted in U.S. Pat. Nos. 5,652,661 and 5,701,185. While such methods offer several advantages in ease of optical design for printing, there are several drawbacks to the use of conventional transmissive LCD technology. Transmissive LCD modulators generally have reduced aperture ratios and the use of transmissive field-effect-transistors (TFT) on glass technology does not promote the pixel to pixel uniformity desired in many printing and film recording applications. Furthermore, in order to provide large numbers of pixels, many high resolution transmissive LCDs posses footprints of several inches. Such a large footprint can be unwieldy when combined with a lens designed for printing or film recording applications. As a result, most LCD printers using transmissive technology are constrained to either low resolution or small print sizes.
To print high resolution images with at least 2000 pixels per inch requires 2000×1500 pixels, and can require as much as 4000×3000 pixels. Transmissive LCD modulators with such resolution are not readily available. Furthermore, each pixel must have a gray scale depth to render a continuous tone print uniformly over the frame size, which is not available with this technology.
The use of a reflective LCD serves to significantly reduce the cost of the printing system. Furthermore, the use of an area reflective LCD modulator sets the exposure times at sufficient length to avoid or significantly reduce reciprocity failure. Exposure times for individual pixels shift from tens of nanoseconds to tens of milliseconds, a million-fold increase. A modest increase in the throughput of the writer to two frames per second will still be well within reciprocity boundaries.
The progress in the reflective LCD device field made in response to needs of the projection display industry have provided opportunities in printing applications. Thus, a reflective LCD modulator designed for projection display can be incorporated into a printing design with little modification to the LCD itself. Also, by designing an exposure system and data path with an existing projection display device allows incorporation of an inexpensive commodity item into the print engine.
Of the reflective LCD technologies, the most suitable to this design is the one that incorporates a small footprint with an integrated CMOS backplane. The compact size along with the uniformity of drive offered by such a device will translate into better image quality than other LCD technologies. There has been progress in the projection display industry towards incorporating a single reflective LCD, primarily because of the lower cost and weight of single device systems. See U.S. Pat. No. 5,743,612. Of the LCD technologies, the reflective LCD with a silicon backplane can best achieve the high speeds required for color sequential operation. While this increased speed may not be as essential to printing as it is for projection display, the higher speeds can be utilized to incorporate additional gray scale and uniformity correction to printing systems.
The recent advent of high resolution reflective LCDs with high contrast, greater than 100:1, presents possibilities for printing that were previously unavailable. See U.S. Pat. Nos. 5,325,137 and 5,805,274. Specifically, a printer may be based on a reflective LCD modulator illuminated by filtered lamps, lasers, or by an array of red, green, and blue light emitting diodes. The reflective LCD modulator may be dithered in two directions to increase the resolution. Thus, a 2000×1500 modulator can be dithered to achieve an image of 4000×3000, equivalent to the best output of the laser writers. Settling times on some modulators are of the order of one-sixtieth of a second or less, allowing possible printing speeds of tens of frames per second.
Reflective LCD modulators have been widely accepted in the display market. Most of the activity in reflective LCD modulators has been related to projection display, such as is disclosed in U.S. Pat. No. 5,325,137. Several projector designs use three reflective LCD modulators, one for each of the primary colors. One such design is shown in U.S. Pat. No. 5,743,610.
It is instructive to note that imaging requirements for projector and display use (as is typified in U.S. Pat. Nos. 5,352,137; 5,808,800; and 5,743,610) differ significantly from imaging requirements for digital printing onto photographic paper or film. Projectors are optimized to provide maximum luminous flux to a screen, with secondary emphasis placed on characteristics important in printing, such as contrast and resolution. To achieve the goals of projection display, most optical designs use high intensity lamp light sources. Optical systems for projector and display applications are designed for the response of the human eye, which, when viewing a display, is relatively insensitive to image artifacts and aberrations and to image non-uniformity. However, when viewing printed output from a high-resolution printing system, the human eye is not nearly as forgiving to artifacts and aberrations and to nonuniformity, since irregularities in optical response are more readily visible and objectionable on printed output. In fact, the gamma of the human eye and brain system, when viewing images in a darkened room is approximately 0.8, rather than a gamma of 1.0 when viewing a print in a well lit room. The gamma associated with a color intermediate negative may be 1.0, and as high as 4.0 on the release print, thus rendering small artifacts more easily visible in high contrast printed images. For this reason, there can be considerable complexity in optical systems for providing uniform exposure energy for printing. Even more significant are differences in resolution requirements. Additionally, projectors are often designed to present motion images. When an image is moving, the presence of defects and artifacts may be dynamic. Between the varying image content and the motion of the artifacts themselves, image variations may not be easily perceived by the human eye. However, when the artifacts are stationary as is the surrounding image data, image quality requirements become more stringent. Digitally rendered movie images, both negative and positive, are typically inspected under a 200× microscope as well as projected onto a screen before approval is given.
A preferred approach for digital printing onto photographic film uses a reflective LCD-based spatial light modulator. Liquid crystal modulators can be a low cost solution for applications requiring spatial light modulators. Photographic printers using commonly available LCD technology are disclosed in U.S. Pat. Nos. 5,652,661; 5,701,185; and 5,745,156. Although the present invention primarily addresses use of reflective LCD spatial light modulators, references to LCDs in the subsequent description can be generalized, for the most part, to other types of spatial light modulators, such as the previously noted Texas Instruments DMD device.
Primarily because of their early development for and association with screen projection of digital images, spatial light modulators have largely been adapted to continuous tone (contone) color imaging applications. Unlike other digital printing and film recording devices, such as the CRT and laser-based devices mentioned above that scan a beam in a two-dimensional pattern, spatial light modulators image one complete frame at a time. Using an LCD, the total exposure duration and overall exposure energy supplied for a frame can be varied as necessary in order to achieve the desired image density and to control media reciprocity characteristics. Advantageously, for printing onto photographic paper and film, the capability for timing and intensity control of each individual pixel allows a LCD printer to provide grayscale imaging.
A single reflective LCD can be used in a color printer by sequentially exposing the LAD to red, green, and blue light while synchronously and sequentially altering the image data and setup parameters sent to the modulator. Such a printer is described in U.S. Pat. No. 6,215,547. The problem with using a single LCD is that the throughput for sequential exposure is lower than simultaneous exposure by a factor of three, the number of exposing colors. The market is trending for the digitization of camera negatives of most feature length movies, not just the special effects portions of a few seconds to a few minutes. Many more frames must then be written out. The speed of the printer in terms of the number of frames per second must increase. The demand for higher speed printers calls for the use of three LCD chips, one for red, one for green, and one for blue. Film frames can then be exposed simultaneously, effectively tripling the printing speed.
It is advantageous to have independent red, green, and blue sources in a writer. LEDs of different colors are typically made of different materials and exhibit different forward voltage drops, requiring separate power supplies. If modest sized (75 watt) tungsten lamps with narrow band filters are used, the power output of the lamps can be individually adjusted to compensate for aging by adjusting the DC lamp current. A change of a few volts in the lamp drive can result in a change of 2:1 in light output. This adjustment can be used to compensate for lamp aging. U.S. Patent application Ser. No. 09/794,669 describes an embodiment where three light sources are used with three reflective LCDs and are contained in a single plane parallel to the mounting tabletop in a low cost paper printer. Depending upon the polarization state used, this writer may be limited in contrast if the alternate (low contrast) side of the beamsplitter prism is used (s-polarized), or may require an expensive, custom x-cube if the nominal high contrast side of the beamsplitter prism is used (p-polarized). JVC LCD-based projectors also confine all the optics to a single plane, perpendicular to the mounting tabletop in this case, but use the high contrast side of the polarizing beamsplitter prism. The p-polarized beam is converted to an spolarized beam in each color, before entering the x-cube, by a half wave plate. These half wave plates must be large and very flat because they are used in the high resolution imaging side of the optical systems and are therefore very expensive. Three are required per projector.
Movie film (intermediate negative) typically requires a dynamic exposure range of 100:1 to achieve a density range of 2.0, which then implies a contrast or extinction ratio of at least 100:1 from the LCD spatial light modulator. Paper prints, with a gamma as high as 2.5, require a dynamic exposure range of only 10:1 or less. Polarizing beamsplitter prisms typically have one side or selected polarization that provides a higher contrast or extinction ratio than the alternate side. It is desirable to use the high contrast side of the polarizing prism for a movie film writer. It is further desirable to have s-polarized light entering the x-cube combiner for all colors, so that a non-custom x-cube can be used and an output analyzer can be used after the x-cube to enhance contrast further. Keeping the optical axis parallel to the tabletop will allow ease of assembly in a low volume package and can be made relatively immune to vibration.
Thus, it is desirable to have a low-cost, high-resolution, high-speed method for digital printing onto a photosensitive media that provides high contrast, low cost standard components, avoids reciprocity failure, is confined to a minimal physical size, rugged, and immune to the effects of vibration.